(1) Field of the Invention
This invention relates to a solid-state image sensor for a Charge-Coupled Device (CCD), in particular to a solid-state image sensor with high pixel density or a solid-state image sensor having miniaturized chips.
(2) Description of the Related Art
Hereafter, a conventional solid-state image sensor is described. FIG. 1A is a cross-section diagram of a conventional interline-type solid-state image sensor. As shown in FIG. 1A, the solid-state image sensor is made up of an n-type silicon substrate 1, a p−−-type well region 2, an n-type charge storage region 3, a p++-type region 4 formed on the n-type charge storage region 3, a p+-type region 5 formed on the n-type charge storage region 3 so as to horizontally surround the p++-type region 4, an n-type buried channel region 7, a p−-type region 6 formed under the n-type buried channel region 7, a read-out gate electrode 8 formed by a polysilicon film and the like, a p+-type signal charge read-out region 9, a p-type region 10, and a light-shielding film 11 such as an aluminum film.
A portion of the p−−-type well region 2 under the n-type charge storage region 3 and the n-type charge storage region 3 form a photodiode unit (photoelectric conversion unit). The p++-type region 4 and the p+-type region 5 restrain dark current. The p−-type region 6 controls smear. The n-type buried channel region 7 forms a charge transfer unit for transferring signal charge which is obtained through photoelectric conversion performed by the photodiode unit. The read-out gate electrode 8 reads out signal charge from the n-type charge storage region 3 to the n-type buried channel region 7 by applying voltage. The p+-type signal charge read-out region 9 controls a read-out voltage. The p+-type region 10 forms a channel stopper. The above is disclosed in Japanese Laid-Open Patent Publication No. 7-142696 and in Japanese Laid-Open Patent Publication No. 10-50976.
In general, due to an increase of intensity of an electric field of a read-out path following a development of micro-pixel size, and an increase of intensity of an electric field between a silicon substrate and a gate insulating film following a development of thinner gate insulating film, hot carriers generated at the time of reading are injected in the gate insulating film causing an increase of the threshold voltage. Consequently, higher read-out voltage is needed with the light irradiation. Therefore, all of the charge is not read with normal voltage and remaining charge is generated. Accordingly, such reliability loss is caused.
In the mentioned conventional solid-state image sensor, in order to prevent the threshold voltage from increasing, an offset (not to be adjacent to each other) is formed between the P+-type region 5 and the n-type buried channel region 7. However, in order to prevent the threshold voltage from increasing with the light irradiation, if the impurity concentration of the P+-type signal charge read-out region 9 is decreased and the read-out voltage is reduced, the surface of the substrate on the top of the p+-type signal charge read-out region 9 which is the offset region is depleted, and dark current is increased in the charge transfer unit.
FIG. 1B shows a potential profile for electrons between X–X′ on the cross-section of the solid-state image sensor shown in FIG. 1A. In the range of X–X′, the section a is a section between the p+-type region 5 and the n-type buried channel region 7, and the section b and c respectively correspond to the p+-type region 5 and the P++-type region 4. As shown in FIG. 1A, in the conventional solid-state image sensor, a boundary (A) of the p+-type region 5 on the side of a signal charge read-out unit matches a boundary (B) of the n-type charge storage region 3 on the side of the signal charge read-out unit. Herein, as shown in the potential profile of FIG. 1B, a potential barrier against the electrons on the impurity concentration boundary (indicating boundary (A) and (B)) d and the remaining charge q is generated. In order to completely read out the remaining charge, it is necessary to increase the read-out voltage.
As a result, the conventional structure cannot simultaneously prevent degradation of image luminance by the remaining charge and generation of noise on the image by the generation of dark current.